- In classical wave theory, electromagnetic (EM) radiation is assumed to behave as a wave
- This is demonstrated by the fact EM radiation exhibits phenomena such as diffraction and interference
- However, experiments from the last century, such as the photoelectric effect and atomic line spectra, can only be explained if EM radiation is assumed to behave as particles
- These experiments have formed the basis of quantum theory, which will be explored in detail in this section
- The energy of a photon can be calculated using the formula:
E = hf
- Using the wave equation, energy can also be equal to:
- E = energy of the photon (J)
- h = Planck's constant (J s)
- c = the speed of light (m s-1)
- f = frequency in Hertz (Hz)
- λ = wavelength (m)
- This equation tells us:
- The higher the frequency of EM radiation, the higher the energy of the photon
- The energy of a photon is inversely proportional to the wavelength
- A long-wavelength photon of light has a higher energy than a shorter-wavelength photon
Light of wavelength 490 nm is incident normally on a surface, as shown in the diagram.
The power of the light is 3.6 mW. The light is completely absorbed by the surface.
Calculate the number of photons incident on the surface in 2.0 s.
Step 1: Write down the known quantities
Wavelength, λ = 490 nm = 490 × 10-9 m
Power, P = 3.6 mW = 3.6 × 10-3 W
Time, t = 2.0 s
Step 2: Write the equations for wave speed and photon energy
Step 3: Calculate the energy of one photon
Step 4: Calculate the number of photons hitting the surface every second
Step 5: Calculate the number of photons that hit the surface in 2 s
(8.9 × 1015) × 2 = 1.8 × 1016
The values of Planck’s constant and the speed of light will always be given to you in an exam, however, it helps to memorise them to speed up calculation questions!